Dark current correction in scinitillator detectors for downhole nuclear applications

ABSTRACT

A radiation logging tool is provided that includes a scintillator detector for use on a wellbore tool string to characterize earth formations. The scintillator detector has a shutter to allow for the collection of data differentiating between incident radiation, such as backscatter signal, and system noise, such as dark current, vibration noise, electronics thermal noise, and electrostatic noise. The radiation logging tool provides for a method of calibrating and measuring incident radiation by the removal of system noise. The shutter is positioned between the photosensor and scintillation member of the scintillator detector, and is able to switch between open and closed states while the scintillation detector is deployed. Measurements of signal noise can be used to calibrate the sampling signal of incident radiation on the scintillator detector.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a U.S. national phase under 35 U.S.C. 371 of InternationalPatent Application No. PCT/US2015/049931, titled “Dark CurrentCorrection in Scintillator Detectors for Downhole Nuclear Applications”and filed Sep. 14, 2015, the entirety of which is incorporated herein byreference.

TECHNICAL FIELD

This disclosure relates to apparatus and systems for nuclear signaldetection and related sensory apparatus for use with a drilling system,or other such well system tool string deployed in hydrocarbon wells andother wells.

BACKGROUND

Photosensitive devices can be used as scintillator detectors fordetecting nuclear signals that are used to evaluate formations forwireline and logging while drilling applications. Dark current isinherently generated by a photosensitive device. Dark current can appearas low level noise and can impact low radiation energy measurements oftools such as density tools. Dark current can increase as temperatureincreases. Some photosensitive devices can be overwhelmed by darkcurrent noise generated at high temperatures. The voltage applied to thephotosensitive device can be increased to compensate for the gain of thephotosensitive device that is lost due to increased temperature orphotosensitive device drift over time. But dark current-generated noisealso increases as the voltage applied to the photosensitive deviceincreases.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative aspects and examples of the present disclosure aredescribed in detail below with reference to the following drawingfigures.

FIG. 1 is a schematic diagram of a well drilling tool string havingnuclear signal detectors deployed in a wellbore, according to someaspects of the present disclosure.

FIG. 2 is a schematic diagram of a radiation logging tool that includesa scintillator detector and a radiation emitter, according to someaspects of the present disclosure.

FIG. 3 is a schematic diagram of an example of a scintillator detectorthat has a check source adjacent to the scintillator detector, accordingto some aspects of the present disclosure.

FIG. 4 is a schematic diagram of another example of a scintillatordetector that has a check source incorporated with the scintillatordetector, according to some aspects of the present disclosure.

FIG. 5 is a schematic diagram of a further example of a scintillatordetector that has an unshielded scintillation crystal arranged in serieswith a shielded secondary scintillation crystal that has a check source,according to some aspects of the present disclosure.

FIG. 6 is a schematic diagram of a further example of a scintillatordetector that has a shielded secondary scintillation crystal with acheck source arranged in series and with an unshielded scintillationcrystal, according to some aspects of the present disclosure.

FIG. 7 is a flowchart of a process for collecting sample signal datafrom a radiation-emitting sensory tool assembly and calibrating thesample signal data to account for device noise, according to someaspects of the present disclosure.

DETAILED DESCRIPTION

A scintillator detector is an instrument used for detecting andmeasuring ionizing radiation, which in well drilling or hydrocarbonwellbore applications can particularly focus on collecting gamma (γ) orneutron radiation. A scintillator detector can include a scintillatorelement, such as the scintillation crystal, that generates photons oflight in response to incident radiation. The photosensor of thescintillation detector can receive photons from the scintillator elementand convert that light into an electrical signal, and often can amplifyit. The scintillator detector can be used with electrical componentsthat can transmit, store, and process the electrical signal. The outputof a scintillator detector can be referred to as counts. Scintillatordetectors can measure both the intensity and the energy of incidentradiation. In wellbore applications, the incident radiation can bebackscatter signal radiation. In particular, a radiation logging tool ona tool string can emit a nuclear radiation signal into the surroundingearth strata, and the emitted signal can reflect, refract, or diffract(e.g., scatter) off of the earth strata or formations in the earthstrata. The amount redirected back toward the tool string is abackscatter signal. The backscatter signal can be a component of astandard sampling signal. Scintillator detectors can characterizeformations in earth strata (e.g., by determining the properties oridentifying the contents of regions) based on spectral data, densityreadings, and the like, derived from the backscatter signal.Scintillator detectors can also be used to sample the naturallyoccurring radiation. In such systems, active logging sources may not beused; however, a reference check source (or seed source) can be used.

Certain aspects of the present disclosure relate to a scintillatordetector with a shutter that is between a photosensor and ascintillation crystal and that can be used to correct for dark currentin downhole nuclear applications. Dark current is electric current fromphotosensitive devices inherently and randomly generating electrons andcharge-holes when no photons are in fact entering the devices. Theshutter can be operable between an open position and a closed positionby a voltage. In the open position, the shutter can allow light producedby the scintillation crystal to pass to the photosensor and be measuredwith the dark current of the photosensor. In the closed position, theshutter can block light from passing to the photosensor, but the darkcurrent of the photosensor can still be measured. The dark current canbe measured by the photosensor for a certain amount of time when theshutter is in the closed position. The measured dark current can besubtracted from light measurements during a normal measurement period,such as when the shutter is in the open position, to remove the darkcurrent component of measurements from the measured light. The measuredlight with the dark current removed can be further processed. Removingdark current or additional types of noise, such as electronic noise anddevice vibration noise, can statistically improve the accuracy andsensitivity of measurements by photosensitive devices.

In some examples, the shutter can be a liquid crystal shutter that canbe activated, in real time, by the voltage to be in the closed position.The output of the photosensor when the shutter is in the closed positioncan represent the counts due to dark current only. In other examples,the output can represent the dark current and other types of systemnoise. The dark current can be sampled for a predetermined period oftime as part of a self-calibration cycle. The measured dark current andother types of measured system noise can then be subtracted for normalmeasurement, when the shutter is in the open position. This can be usedfor bulk count or spectral measurements.

A drill tool can have other sources of noise that interferes withsignals collected by sensors on the tool string. Electronic equipmentpresent on the tool string generates electronic noise, such as thermalnoise, flicker noise, interference, crosstalk, and the like. Variousaspects of the present disclosure may be used in wireline or slicklinesampling operations, permanent or semi-permanent production monitoring,logging while drilling (LWD) applications, or measurement while drilling(MWD) applications. In LWD and MWD applications, the vibration of thetool string can generate a vibration noise, which can includeelectrostatic effects, the movement of wires and correspondingelectromagnetic effects, and signals triggered by the physical motionalone. Contact between the tool string and the walls of a wellbore canalso be a source of vibration noise. The dark current, electric noise,and vibration noise can be characterized as system noise.

A process to remove noise resulting from any or all of dark current,electronic noise, and vibration noise from a standard sampling signalcan include a calibration process. The calibration process can beperformed as a cycle in between standard signal sampling of ascintillator detector such that the calibration process is a real-timecalibration process. By closing a shutter in the scintillator detectorfor a calibration cycle, the photosensor coupled to the scintillatorelement can measure the system noise affecting the device. In someaspects, the calibration cycle can be performed over a range of timethat can be milliseconds in scale, seconds in scale, or aggregatesthereof. The system noise can be removed from the sampling signal ofincident radiation collected when the shutter is open.

The illustrative examples discussed herein are given to introduce thereader to the general subject matter discussed here and are not intendedto limit the scope of the disclosed concepts. The following sectionsdescribe various additional aspects and examples with reference to thedrawings in which like numerals indicate like elements, and directionaldescriptions are used to describe the illustrative aspects. Thefollowing sections use directional descriptions such as “uphole” and“downhole” in relation to the illustrative aspects as they are depictedin the figures, the uphole direction being toward the surface of thewell, the downhole direction being toward the toe of the well. Thelongitudinal axis of the tool string can be referred to as the“centerline” of the tool string. Further, portions of structuralelements described herein can be referred to by their generalorientation when deployed, e.g. an uphole end or downhole end. Like theillustrative aspects, the numerals and directional descriptions includedin the following sections should not be used to limit the presentdisclosure.

FIG. 1 is a schematic diagram of a well drilling system 100 having atool string 106 deployed in a wellbore 102. The tool string 106 can havesensors. In the well drilling system 100, a wellbore 102 formed in earthstrata 104 is drilled by rotating a drill bit 118 coupled to the toolstring 106. In some aspects, a well casing 105 can be set and arrangedalong the sides of earth strata 104 defining the wellbore 102. In otheraspects, a well drilling system 100 can be applied in an open holewellbore.

The tool string 106 can include a mandrel 108 with one or more radiationlogging tools 110. The radiation logging tools 110 can be positionedalong the mandrel 108, uphole or downhole of other radiation loggingtools 110. In some aspects, other sensors, tools, communicationapparatus, instrumentation, and other tool string apparatuses (notshown) can be used in well drilling systems anywhere along the mandrel108 up to the well surface 103 or down to the bottom (or toe) of thewellbore 102. Each radiation logging tool 110 can be mechanicallycoupled to the mandrel 108 extending radially from the centerline of themandrel 108, or positioned on components other than the mandrel 108.Each radiation logging tool 110 can include a radiation emitter thatemits nuclear signal into the earth strata 104 that scatter offformations 107 within the earth strata. Each radiation logging tool 110can also include a scintillator detector that has components formeasuring the backscatter signals and removing dark current and othernoise from the measured signals.

FIG. 1 further shows a drilling tool having a drill bit 118 andradiation logging tools 110 can be operated during drilling (e.g.,taking LWD/MWD recordings) and during periods where the drill bit 118 isnot actively drilling or rotating within the wellbore 102. The controlunit 120 can be located at the surface 103 of the well drilling system100. The control unit 120 can include a non-transitory computer-readablemedium and microprocessors configured to receive data from the radiationlogging tools 110 located along the tool string 106. In some aspects,the control unit 120 can further control the operation of the toolstring 106 and the drill bit 118, or any other apparatus, tool, orinstrumentation coupled to the tool string 106. In some aspects, thecontrol unit 120 can collect or aggregate data received from theradiation logging tools 110 and generate a profile or characterizationof the earth strata 104 and formations 107 proximate to the wellbore102. The control unit 120 can further include a user interface to allowfor an operator to monitor and control the function of the tool string106, and to evaluate any measurements of signals received from theradiation logging tools 110 or other sensory apparatus located downhole.

FIG. 2 is a schematic diagram of the radiation logging tool 110according to one example. The radiation logging tool 110 includes ascintillator detector 201 and a radiation emitter 203 for emittingnuclear signals 213. The scintillator detector 201 can include aphotosensor 202, a shutter element 204, and a scintillation crystal 206.The radiation emitter 203 can be located downhole with the scintillatordetector 201 and can emit nuclear signals 213 into the surroundingformations. The signals 213 can reflect, refract, or diffract off ofearth strata, as well as formations in the earth strata, as backscattersignals that can be a component of incident signals 214. The incidentsignals 214 can be received by the scintillation crystal 206 of thescintillator detector 201, leading the scintillation crystal 206 togenerate photons. In some aspects, natural gamma radiation of thesurrounding earth strata and formations can also be a component of theof incident signals 214. A direct shielding element 205 can bepositioned between the radiation emitter 203 and the scintillatordetector 201 such that nuclear signals 213 are generally prevented frombeing directly received by the scintillation crystal 206. Thus, thescintillator detector 201 can collect and measure backscatter signalscharacterizing the earth strata without noise or interference causedfrom inadvertent collection of direct nuclear signals 213 from theradiation emitter 203.

In alternative aspects, the radiation logging tool 110 can beconstructed without a radiation emitter 203, such that the scintillatordetector 201 collects incident signals 214 primarily based on naturalgamma radiation.

The shutter element 204 is positioned between and coupled to thescintillation crystal 206 and the photosensor 202. Light can be blockedfrom entering the photosensor 202 from the scintillation crystal 206when the shutter element 204 is in a closed configuration. Light can beallowed to pass to the photosensor 202 when the shutter element 204 isin an open configuration. The shutter element 204 can be driven by ashutter control voltage 210 to actuate the shutter between an openconfiguration and a closed configuration. The scintillator detector 201can log sample signals based on incident signals 214 while the shutterelement 204 is in the open configuration. The scintillator detector 201can log signals in the photosensor 202 based on system noise (includingdark current) while the shutter element 204 is in the closedconfiguration, which can be during a calibration cycle, for example. Anoutput signal 212 is generated by the photosensor 202, and thescintillator detector 201 transmits the output signal 212 to otherelectronics, such as the local processing unit 207 or a control unit atthe surface of the wellbore, for processing, manipulation, and analysisof the collected output signal 212.

The shutter element 204 can be a liquid crystal shutter, a mechanicalshutter, an optical shutter, a piezoelectric, an optical grating, orother such element that can be actuated between an open and a closedconfiguration. In aspects where the shutter element 204 is a liquidcrystal shutter, the liquid crystal shutter can turn opaque or black andblock light when the shutter control voltage 210 is applied to theshutter element 204. Applying shutter control voltage 210 to the shutterelement 204 can be controlled and executed at any time while thescintillator detector 201 is deployed downhole. In aspects where theshutter element 204 is a mechanical shutter or other optical shutter,the aperture of the shutter element can be opened, closed, or cycledwhen the shutter control voltage 210 is applied to the shutter element204. In aspects where the shutter element 204 is a piezoelectric or anoptical grating, the optical path of light from the scintillationcrystal 206 can be shifted toward or away from the photosensor 202 whenthe shutter control voltage 210 is applied to the shutter element 204. Aparticular orientation of the piezoelectric or optical grating elementas a shutter element 204 can operate as a closed configuration, even ifthe optical path does continue to pass through the piezoelectric oroptical grating element.

The photosensor 202 can be supplied power at an operational voltage 208sufficient to power elements of the photosensor 202 to collect signalsfrom the scintillation crystal 206. In some aspects, the power of theoperational voltage 208 driving the photosensor 202 can be from a powersource coupled to or within a local processing unit 207. The photosensor202 can be a photomultiplier tube (PMT), and the operational voltage 208can be sufficient to maintain the relative voltages of dynodes of theelectron multiplier within the PMT. In alternative aspects, thephotosensor 202 can be a photodiode, an avalanche photodiode, solidstate photomultiplier, or other such semiconductor device that cangenerate an output signal 212 correlated to the incident signal 214. Theoutput signals 212 can be stored, processed, and transmitted by thelocal processing unit 207 that is electronically coupled to thescintillator detector 201.

The local processing unit 207 can include a processor and anon-transitory computer-readable medium. The local processing unit 207can receive, store, process, and transmit data and signals such asoperational instructions from a control unit located elsewhere in thewell drilling system. The local processing unit 207 can further includecomputer-executable instructions or algorithms to process, convert,transform, or otherwise manipulate data or signals received from thescintillator detector 201. The local processing unit 207 can transmitsignals as output signals 212 to the control unit (either directly orindirectly through the bus master), or other data processing module. Theoutput signals 212 can be transmitted as a continuous stream of data, asbatches or packages of data, or in any other transmission scheme. Inother aspects, the local processing unit 207 of a given radiationlogging tool 110 can be electronically coupled to the local processingunits 207 of other radiation logging tools 110 on a given tool stringand can transmit signals through the other radiation logging tools 110for processing.

In some aspects, the well drilling system can include a downholecomputer (referred to as a “bus master”) that is electronically coupledto one or more tools along the tool string and also to a control unit120 at the surface of the well. The downhole computer (not shown) caninclude a microprocessor having a non-transitory computer-readablemedium that further includes computer-executable instructions oralgorithms to store, process, convert, transform, or otherwisemanipulate data or signals received from one or more radiation loggingtools 110. Data received by the downhole computer from one or moreradiation logging tools 110 can be ultimately relayed to the controlunit 120.

The local processing unit 207 can correct the output signals 212 derivedfrom standard signal sampling by removing system noise collected duringone or more calibration cycles. During a calibration cycle the shutterelement 204 is closed, and the output signal 212 is representative ofsystem noise, which includes noise inherent to the photosensor 202 andscintillator detector 201. The local processing unit 207 can calculateand store a correction value based on the output signals 212 collectedduring a calibration cycle. The correction value can be an average ofthe output signals 212 collected during a calibration cycle, a medianvalue of the output signals 212 collected during a calibration cycle, acombination of the output signals 212 collected during two or morecalibration cycles, a combination thereof, or another statisticallycalculated value. During standard signal sampling, the shutter elementis open, and the output signals 212 are representative of the incidentsignals 214 triggering photons that transmit through the scintillationcrystal 206. During standard signal sampling dark current inherent tothe photosensor 202 and other system noise are also part of the outputsignals 212. The local processing unit 207 can apply the correctionvalue determined from one or more calibration cycles to the outputsignals 212 collected during standard signal sampling to remove darkcurrent (and other types of system noise) to produce a calibrated set offormation data. The calibrated set of formation data can more accuratelyrepresent the incident signals 214.

In some aspects, the calibrated set of formation data can be stored fora period of time on the non-transitory, computer-readable medium locatedin the local processing unit 207 and subsequently transmitted as astream of data or as a data package to a control unit electronicallycoupled to the radiation logging tool 110. In other aspects, oncecalculated by the local processing unit 207, the calibrated set offormation data can be directly transmitted in real-time to a controlunit electronically coupled to the radiation logging tool 110. Infurther aspects, the calibrated set of formation data can be stored fora period of time on the non-transitory, computer-readable medium locatedin the downhole computer and subsequently transmitted as a stream ofdata or as a data package to the control unit 120. The calibration datacan also be used as diagnostic value to determine the nominal noise ofthe radiation logging tool 110 or the operational life of the radiationlogging tool 110.

A calibration cycle can be initiated according to external stimulus,such as a threshold temperature of the surrounding environment. In otheraspects, the calibration cycle can be initiated manually or directly byan operator through a control interface at the surface of the wellbore.In further aspects, the calibration cycle can be initiated according toan automatic timing sequence, where the timing sequence can be at equalintervals or in a pattern of timing intervals.

The scintillation crystal 206 can be made from any suitable material.Examples of suitable materials include inorganic crystals based on,though not limited to, cesium iodide (CsI), sodium iodide (NaI), sodiumiodide doped with thallium (NaI:Tl), zinc sulphide (ZnS), lithium iodide(LiI), or bismuth germinate (BiGeO). The scintillation crystal 206 canfurther include other elements as a dopant in the crystal structure tomodify the electrical characteristics of the scintillation crystal 206.Inorganic scintillation crystals can be selected for use in specificapplications. Cesium iodide in crystalline form can be used as ascintillator for the detection of protons and alpha (α) particles. Zincsulphide can also be used as a detector of alpha (α) particles. Sodiumiodide containing an amount of thallium as a dopant can be used as ascintillator for the detection of gamma (γ) waves. Lithium iodide (LiI)can be used as a detector for neutrons. Other examples of suitablematerials for the scintillation crystal 206 include transparentinorganic crystals made of anthracene, stilbene, naphthalene, or otheraromatic hydrocarbons.

In some aspects, a transparent silicon-based gel pad or grease (notshown) can be located between the photosensor 202 and scintillationcrystal 206. The pad or grease pad can provide cushioning and reducehard impact between the elements of the scintillator detector 201 causedby physical motion, such as the vibration of a drill bit at the end of adrill string.

The radiation emitter 203 can have any one of several sources foremitting nuclear signals 213. In some aspects, the radiation emitter 203can have an elemental isotope source, which can be used to emit gamma orneutron radiation into the earth strata of a wellbore. In some aspects,the element or isotope can include, but is not limited to, a cesium (Cs)source, a cobalt (Co) source, or an americium source (Am). The elementsand isotopes of elements used for the radiation emitter 203 can have anemission with an amount of energy from about tens of kiloelectrovolts toabout tens of megaelectrovolts. In other examples, the radiation sourcecan produce gamma radiation having an energy of about sixtykiloelectrovolts (60 keV). Earth strata can also have a component ofnatural gamma radiation that can be collected as incident signals 214 bythe scintillator detector 201. In alternative aspects, the radiationemitter 203 can emit neutron radiation, or the radiation emitter 203 canbe a spectral tool.

Scintillator detectors according to other examples can include a sourcefor a reference signal by which the effectiveness of removing darkcurrent or other types of system noise can be checked. FIG. 3 is aschematic diagram of another example of the scintillator detector 301.The scintillator detector 301 includes the same components as thescintillator detector 201 in FIG. 2, but it also includes a check source216 adjacent to the scintillation crystal 206. Similarly, FIG. 4 is aschematic diagram of a further example of a scintillator detector 401that includes the same components as the scintillator detector 201 shownin FIG. 2, but it includes the check source 216 incorporated within thescintillation crystal 206. The check source 216 can provide foradditional gain stabilization of the photosensor 202.

For example, the check source 216 can be used in combination with ascintillation crystal 206 to provide a reference signal 218. The checksource 216 can emit a small, trace amount of radioactivity proximate tothe scintillation crystal 206 as in FIG. 3, or incorporated as part ofthe scintillation crystal 206 as in FIG. 4. The reference signal 218provided by the check source can cause the scintillation crystal 206 togenerate photons. The check source 216 can emit the reference signal 218at a lower energy level than the backscatter signal that the respectivescintillator detector 301, 401 is configured to detect. In some aspects,however, the check source 216 emits a reference signal 218 that has anenergy level in the same range as dark current emissions. Removing darkcurrent noise from the output signals 212 can be confirmed by theremoval of an expected reference signal 218 from the check source 216.

The check source 216 can contribute to gain stabilization of thephotosensor because the reference signal 218 has a known and stable peakenergy, and deviation from that energy level can be used to determinethe degree of drift, if any, in a photosensor 202. For applicationswhere the incident signal 214 can have a relatively high energy level,the check source 216 can be selected to emit a relatively low energylevel reference signal 218 distinguishable from the expected incidentsignals 214. If a component of the output signals 212 that is based onthe photons triggered by the reference signal 218 indicates drift of thephotosensor 202, the operational voltage 208 of the photosensor can beadjusted to alter and stabilize the gain of the photosensor 202.

The check source 216 may emit a reference signal 218 in the same energylevel range as expected from the dark current. The reference signal 218can thereby provide a baseline signal from which the dark currentcomponent of the output signals 212 can be calculated. In some aspects,the operational voltage 208 of the photosensor 202 does not need to beincreased solely to compensate for a decreased gain. The photosensor 202can thus be operated at a relatively lower average operational voltage208 over the course of the photosensor operational life, indirectlyextending the operational life of the photosensor 202 by slowing therate of degradation of the photosensor 202. In some aspects, the checksource 216 can include an isotope of an element, such as americium (Am),cobalt (Co), or Cesium (Cs).

Scintillator detectors according to various examples can includeadditional components. FIG. 5 is a schematic diagram of another exampleof a scintillator detector 501. The scintillator detector 501 is similarto the scintillator detector 401 shown in FIG. 4, except that thescintillation crystal 206 is arranged in series with a shieldedsecondary scintillation crystal 222 that includes the check source 216.The scintillator detector 501 can also further include a secondaryshutter element 220 coupled via an optical fiber 230 to the secondaryscintillation crystal 222 and a shield case 224 directly coupled aroundthe secondary scintillation crystal 222. The shield case 224 cansurround the secondary scintillation crystal 222, except for the portionof the secondary scintillation crystal 222 coupled to the optical fiber230 that is further optically coupled to the secondary shutter element220. Optionally, a lens 228 can be located adjacent to the secondaryscintillation crystal 222 within the shield case 224. The opticallycoupled lens 228 can focus light generated within the secondaryscintillation crystal 222 toward the optical fiber 230. When thesecondary shutter element 220 is in an open configuration, photonsgenerated in the secondary scintillation crystal 222 can pass throughthe optical fiber 230 and the secondary shutter element 220 toward thephotosensor 202. The scintillation crystal 206 can be an unshieldedscintillation crystal in this example. In further alternative aspects,an optional secondary lens (not shown) can be placed between the opticalfiber 230 and the secondary shutter element 220 to diverge the lightreceived via the optical fiber 230 (reversing the focusing function ofthe lens 228) and provide the light to a section of the scintillationcrystal 206 wider than the diameter of the optical fiber 230. Theoptional secondary lens can be located either between the secondaryshutter element 220 and the scintillation crystal 206, or between thesecondary shutter element 220 and the optical fiber 230.

The reference signal 218 from the check source 216 can be selected tohave a minimal number of photons effect the scintillation crystal 206.Minimizing any unintended effect on the scintillation crystal 206 by thereference signal 218 can be accomplished by placing the shield case 224,secondary scintillation crystal 222, and check source 216 at a distancefrom the other components of the scintillator detector 501 and routingthe reference signal 218 through the optical fiber 230 to the secondaryshutter element 220. In such aspects, the secondary scintillationcrystal 222 can be positioned at a distance one or more foot (1 ft.)away from the check source 216, or far enough such that the counts fromthe check source have a minimal impact on the scintillation crystal 206.

Light can be blocked from entering the photosensor 202 from theunshielded scintillation crystal 206 when the shutter element 204 is ina closed configuration. Independently, light can be blocked fromentering the photosensor 202 from the shielded secondary scintillationcrystal 222 (via the unshielded scintillation crystal 206) when thesecondary shutter element 220 is in a closed configuration, therebycontrolling the times at which a reference signal 218 from the checksource 216 passes through unshielded scintillation crystal 206 towardthe photosensor 202. The aperture of the shutter element 204 can beopened, closed, or cycled when the shutter control voltage 210 isapplied to the shutter element 204. Similarly, the aperture of thesecondary shutter element 220 can be opened, closed, or cycled when asecondary shutter control voltage 226 is applied to the secondaryshutter element 220. Both the shutter control voltage 210 and thesecondary shutter control voltage 226 can be driven by a power sourcelocated in the radiation logging tool housing the scintillator detector501, where the power source can be within a local processing unit 207.

In some aspects, the unshielded scintillation crystal 206 and theshielded secondary scintillation crystal 222 can be constructed of thesame scintillation crystal materials. In other aspects, the unshieldedscintillation crystal 206 and the shielded secondary scintillationcrystal 222 can be constructed of different scintillation crystalmaterials. In further aspects, a transparent silicon-based gel pad orgrease (not shown) can be located between the photosensor unshieldedscintillation crystal 206 and the shielded secondary scintillationcrystal 222 to provide cushioning and reduce hard impact between theelements of the scintillator detector 501 caused by physical motion orvibration.

In further aspects, the shield case 224 surrounding the shieldedsecondary scintillation crystal 222 can be made of a high-mass elementthat blocks the reference signal 218 from passing though the shield case224 to prevent the reference signal 218 from directly stimulating thegeneration of photons in the unshielded scintillation crystal 206 whenthe secondary shutter element 220 is closed. Similarly, in some aspects,the shield case 224 surrounding the shielded secondary scintillationcrystal 222 can be made of a high-mass element that blocks incidentsignal 214 from passing though the shield case 224. The high-masscharacteristic of the shield case 224 can prevent the incident signal214 from producing photons in the secondary scintillation crystal 222that are not based on the check source 216. In some aspects, thehigh-mass element used to construct the shield case 224 can be lead (Pb)or another element having sufficient mass to block both the referencesignal 218 and an expected incident signal 214. In other aspects, thehigh-mass element used to construct the shield case 224 can have athickness to provide a total mass sufficient to block both the referencesignal 218 and an expected incident signal 214.

FIG. 6 is a schematic diagram of another example of a scintillatordetector 601. The scintillator detector 601 is similar to thescintillator detector 501 as shown in FIG. 5, except that the secondaryscintillation crystal 222 with the check source 216, shielded by shieldcase 224 is positioned closer to the photosensor 202 than the shutterelement 204 and the unshielded scintillation crystal 206 that receivesincident signals 214 radiation from the formation and earth strata. Inthis configuration, when the shutter element 204 is a closedconfiguration, only the reference signal 218 from the check source 216and system noise (including dark current from the photosensor 202) iscollected. The output signals 212 collected when the shutter element 204is in the closed configuration can be used for stabilization and can besubtracted from the signal data collected when the shutter element 204is open. This configuration allows for subtraction of the backgroundsignals due to the check source 216.

FIG. 7 is a flowchart of a process for collecting sample signals datafrom a radiation-emitting sensory tool assembly, and calibrating thesample signals data to account for system noise. In some aspects, thesensory tool assembly collects logging data during both standard signalssampling phases and calibration cycle phases, and uses logging datacollected during calibration cycle phases to apply a calculatedcorrection to the logging data collected during standard signalssampling. Initially, a tool string having a drilling apparatus isdeployed in a wellbore. The tool string can include a radiation loggingtool, such as the radiation logging tool 110 of FIG. 2. In block 700,the process starts, where the signal to be detected can be from eitheror both of radiation logging tool itself and naturally occurringradiation. In block 702, the radiation logging tool is controlled tooperate a shutter element of a scintillator detector in an openconfiguration or a closed configuration.

In block 704, the shutter is operated in the open configuration anddetected signal, such as sample signals, are collected from incidentsignals received from earth strata and formations surrounding thewellbore in which the assembly is deployed. For example, the radiationemitter of the radiation logging tool emits radioactive signals into thesurrounding wellbore. The direct signals emitted can reflect, refract,or deflect off of the earth strata and formations of the surroundingwellbore. A portion of the backscatter signal, which can be a componentof incident radiation, can be received by the scintillation sensors. Theradiation logging tool can emit signals at any time, or at all times,while the drilling apparatus is deployed. The scintillator detector ofthe sensory tool assembly is receptive to incident signals, which can bean individual signal or a combined signal of backscatter signal, naturalgamma radiation, or natural neutron radiation. The scintillation crystalof the scintillator detector emits photons in response to receivingincident signals. A portion of the photons generated in thescintillation crystal enter the photosensor and thereby are measured andrecorded as counts at the local processor unit.

The logging data stored in the local processor unit can be processed,which in some aspects can be the application of a correction value tothe sample signal data, where the sample signals data is subsequentlytransmitted to or read by the control unit. In some aspects, operationof the scintillator detector with shutters open can continue for apredetermined period of time. In other aspects, the operation of thescintillator detector with shutters open can be controlled by anoperator via a control unit remote from the sensory tool assembly. Invarious aspects, sample signals data from the local processor unit canbe streamed in real-time to the control unit, transmitted in datapackages or batches to the control unit, or sent to the control unit inas a combination of both real-time and packaged data transmissions. Inother aspects, sample signals data from the local processor unit can berelayed through a downhole computer before being transmitted to thecontrol unit.

In block 706, the scintillator detector, with the shutter elementclosed, collects signals representative of system noise that includesdark current. For example, while the radiation logging tool may emitradioactive signals into the surrounding wellbore, the photosensor doesnot receive any photons generated in the coupled scintillation crystalbecause the shutter element of the scintillator detector is closed.Accordingly, during the calibration cycle, the photosensor is in aconfiguration where the photosensor detects system noise, whilemeasuring and recording the dark current, vibration noise, andelectrostatic noise as counts at the local processor unit.

In block 708, the local processor unit calculates a correction valuebased on the collected system noise. The correction value can berepresentative of the vibration noise, electrostatic noise, and anyother system noise of the tool string in addition to dark currentgenerated by the photosensor elements of the scintillation sensors. Inblock 710, the local processing unit can apply the correction value tothe sample signals (from block 704) that are detected when the shutterelement is open.

The data collected during sample signals collection in block 704 can befurther differentiated according to whether the logs were collectedwhile the tool string drilling apparatus was in the process of drillingor not. Similarly, the system noise signals data collected during acalibration cycle in block 706 can be further differentiated accordingto whether the logs were collected while the tool string drillingapparatus was in the process of drilling or not. The system noise willbe different when the tool string drilling apparatus is in the processof drilling or not at least due to the amount of vibration noisepresent. The local processing unit can calculate and apply a differentcorrection value to sample signals depending on whether or not thedrilling apparatus was active at the time of logging.

In block 712, a corrected data set of sample signal processed to removesystem noise from the data is provided or transmitted to a processingdevice, such as a control unit located at the surface of the wellbore.The corrected data set, which can be representative of thecharacteristics of the earth strata and formations surrounding thewellbore, can be further processed and communicated for analysis of theearth strata and formations surrounding the wellbore.

In block 714, the correction value determined in block 708 can be usedto produce a diagnostic value to determine the nominal noise of a givenradiation logging tool or the operational life of a given radiationlogging tool.

In other examples, collecting detected signals in block 704 alsoincludes a signal from a check source that is used to stabilize the gainof the scintillator detector.

Following completion of either a standard sampling cycle or acalibration cycle, the sensory tool process can return to block 702periodically, cyclically, in response to a physical stimulus, inresponse to a programmed sequence, or in response to a user-initiatedcommand.

The subject matter of aspects and examples of this disclosure isdescribed here with specificity to meet statutory requirements, but thisdescription is not necessarily intended to limit the scope of theclaims. The claimed subject matter may be embodied in other ways, mayinclude different elements or steps, and may be used in conjunction withother existing or future technologies. Throughout this description forthe purposes of explanation, numerous specific details are set forth inorder to provide a thorough understanding of examples and aspects of thesubject matter disclosed herein. It will be apparent, however, to oneskilled in the art that the many examples or aspects may be practicedwithout some of these specific details. In some instances, structuresand devices are shown in diagram or schematic form to avoid obscuringthe underlying principles of the described examples or aspects. Thisdescription should not be interpreted as implying any particular orderor arrangement among or between various steps or elements except whenthe order of individual steps or arrangement of elements is explicitlydescribed.

The foregoing description of the disclosure, including illustratedaspects and examples has been presented only for the purpose ofillustration and description and is not intended to be exhaustive or tolimit the disclosure to the precise forms disclosed. Numerous differentmodifications, adaptations, and arrangements of the components depictedin the drawings or described above, as well as components and steps notshown or described, are possible. Similarly, some features andsubcombinations are useful and may be employed without reference toother features and subcombinations. Examples and aspects of the subjectmatter have been described for illustrative and not restrictivepurposes, and alternative examples or aspects will become apparent tothose skilled in the art without departing from the scope of thisdisclosure. Accordingly, the present subject matter is not limited tothe examples or aspects described above or depicted in the drawings, andvarious embodiments, examples, aspects, and modifications can be madewithout departing from the scope of the claims below.

That which is claimed is:
 1. A system for measuring and processingnuclear radiation data collected within a wellbore, comprising: a toolstring; and a radiation logging tool mounted to the tool string, theradiation logging tool including (i) a radiation emitter operable toemit a nuclear signal into earth strata and (ii) a scintillator detectorto collect backscatter signals from earth strata, the scintillatordetector including a shutter operable to actuate between an openconfiguration and a closed configuration, wherein the shutter in theopen configuration allows the scintillator detector to collect signaldata based on the backscatter signals, and the shutter in the closedconfiguration allows the scintillator detector to collect system noisedata to remove system noise from the signal data.
 2. The systemaccording to claim 1, further comprising a local processing unitcomprising a non-transitory, computer-readable medium havinginstructions that are executable for removing system noise from thesignal data to output corrected signal data, the system noise includingdark current.
 3. The system according to claim 2, wherein the localprocessing unit includes instructions for calculating a correction valuebased on the system noise and instructions for using the correctionvalue to remove the system noise from the signal data.
 4. The systemaccording to claim 2, further comprising a remote control unit,electronically coupled to the local processing unit, the remote controlunit being operable to receive signal data from the local processingunit.
 5. The system according to claim 2, wherein the scintillatordetector further comprises a check source coupled to the scintillatordetector for emitting a reference signal that is collected by thescintillator detector.
 6. The system according to claim 5, wherein thelocal processing unit includes instructions for calculating aphotosensor drift of the scintillator detector based on a check sourcereference signal, and for adjusting a power delivered to thescintillator detector based on the calculated photosensor drift.
 7. Thesystem according to claim 5, wherein the local processing unit includesinstructions for calculating a photosensor drift of the scintillatordetector based on a check source reference signal, for calculating anadjustment to the system noise based on the calculated photosensordrift, and for calculating a correction value based on the adjustment tothe system noise for removing from the signal data.
 8. A radiationlogging tool, comprising: a radiation emitter; a scintillator detectorcomprising: a scintillation member operable to receive nuclear radiationsignals from or through an earth formation and to generate photons inresponse to receiving the nuclear radiation signals; a photosensor forreceiving the photons from the scintillation member based on the nuclearradiation signals and for converting the photons to output signals; anda shutter (i) operable between an open configuration and a closedconfiguration to remove system noise from the output signals and (ii)positioned between the scintillation member and the photosensor.
 9. Theradiation logging tool according to claim 8, wherein the photosensor isa photomultiplier tube, wherein the system noise includes dark current.10. The radiation logging tool according to claim 8, wherein the shutteris a liquid crystal shutter, a mechanical shutter, or an opticalshutter.
 11. The radiation logging tool according to claim 8, whereinthe shutter is operable to actuate between the open configuration, thatallows the scintillator detector to collect signal data based onbackscatter signals, and the closed configuration, that allows thescintillator detector to collect system noise data to remove from thecollected signal data, by a shutter control voltage.
 12. The radiationlogging tool according to claim 8, further comprising a shieldingelement positioned between the radiation emitter and the scintillatordetector.
 13. The radiation logging tool according to claim 8, furthercomprising a check source located adjacent to the scintillation member.14. The radiation logging tool according to claim 8, further comprisinga check source located within the scintillation member.
 15. Theradiation logging tool according to claim 12, further comprising: asecondary shutter operable between an open configuration and a closedconfiguration, coupled to the scintillation member; a shield case thatsurrounds a secondary scintillation member, except for a region of thesecondary scintillation member coupled to the secondary shutter, whereina check source is located within the secondary scintillation member; andan optical fiber that optically couples the secondary shutter and thesecondary scintillation member.
 16. The radiation logging tool accordingto claim 15, further comprising a lens optically coupled to andpositioned between the secondary scintillation member and the opticalfiber.
 17. A method of operating a radiation logging tool in a wellbore,comprising: emitting a nuclear signal with a radiation emitter intoearth strata; actuating a shutter element, coupled to and positionedbetween a photosensor and a scintillator detector, between an openconfiguration and a closed configuration; with the shutter element inthe open configuration, logging sample signal data based on incidentsignal with the photosensor; with the shutter element in the closedconfiguration, logging dark current with the photosensor; calculating acorrection value based on the dark current logging; and applying thecorrection value to the sample signal data to remove a component of darkcurrent from the sample signal data.
 18. The method according to claim17, further comprising logging reference signal data concurrent withsample signal data where the shutter element of the scintillatordetector in an open configuration.
 19. The method according to claim 17,further comprising logging vibration noise and electrostatic noiseconcurrent with dark current logging where the shutter element of thescintillator detector is in the closed configuration, and calculatingthe correction value based on the dark current, vibration noise, andelectrostatic noise logging data.
 20. The method according to claim 17,further comprising generating a data set indicative of signals receivedexternal to the scintillator detector and transmitting that data set toa remote control unit.